Internal combustion engines, and particularly diesel or compression-ignition engines, characteristically contain some level of particulates in their exhaust gas effluent. This particulate may include fine droplets of liquid, partially burned fuel and solid particles such as carbon. Further, particulates which are sufficiently large and translucent or opaque as to be visible to the human eye are typically referred to as "smoke". The existence of such particulates in the form of smoke may be objectionable for a variety of reasons, including aesthetics. Accordingly, various governmental agencies and/or automobile and truck manufacturers have imposed or may be expected to impose, limits on the quantity or level of smoke or particulates emitted by a particular engine. As used in the present application, the phrases "particulates" and "smoke" will be used interchangeably.
In the laboratory, various systems exist for identifying and/or quantifying the level of smoke emitted by an internal combustion engine. In one such system a standard volume of exhaust gas is passed through a standard fabric patch and the resulting degree of darkening or opacity of the patch as a result of the exhaust smoke is measured optically and expressed in units of Bosch. Although such measurements are typically made in a test cell on a batch basis, it will be appreciated that continuously operative sensing systems of an optical, electrostatic or other similar nature may be calibrated to provide an indication of smoke or particulate level expressed in units of Bosch or other suitable reference units. One such apparatus for measuring the level of particulates in the exhaust gas emissions from an internal combustion engine that is suitable for on-vehicle applications is disclosed in U.S. Pat. No. 4,456,883 entitled "Method And Apparatus For Indicating An Operating Characteristic Of An Internal Combustion Engine" by Bullis et al., which application was filed on Oct. 4, 1982 and issued on June 26, 1984 and is assigned to the assignee of the present application. Although not limited to diesel or compression ignition engines, considerations of exhaust smoke levels are more prevalent with such engines because of their characteristically higher levels of emitted particulates.
Numerous control systems have been developed for regulating the quantity of fuel admitted to an internal combustion engine and particularly to diesel engines. More specifically the fuel quantity control or metering member, such as a rack, of the diesel pump is variably limited, or stopped, in accordance with a control schedule. Generally speaking a particular maximum fuel flow or fuel flow limit is determinative of a corresponding torque limit for the engine. Thus, as used herein, the terms "torque limit" and "fuel flow limit" or "maximum fuel limit" will be considered to be synonymous. One example of a system for controlling the adjustment of a quantity-metering member of a fuel injection pump is disclosed in U.S. Pat. No. 4,357,920 wherein various operating parameters of a diesel engine are monitored and used to develop a signal which determines the instant limit of the fuel quantity-metering member. One of the values which may serve to limit the fuel quantity injected is a so-called "smoke characteristic" determined as a function of the speed, the fuel temperature and the taken-in quantity of air, the latter value being determined by the measurement of pressure and temperature in the intake manifold or by a direct measurement of the air quantity.
In yet another system disclosed in U.S. Pat. No. 4,502,437 for "Electrical Fuel Control System And Method For Diesel Engines" by James R. Voss, issued Mar. 5, 1985 and assigned to the same assignee as the present application, a system is disclosed for limiting the positioning of the fuel quantity-metering member (rack) in a manner intended to prevent or minimize puffs of smoke which may be emitted during gear-shifting of a turbocharged diesel engine. In that instance, two functions characteristic of the engine and turbocharger combinations being controlled are stored in a puff control memory and are used for the mathematical derivation and provision of a fuel quantity limiting signal as a function of engine speed and the actual positioning of the fuel-metering rack.
Another U.S. Pat. No. 4,223,654 for "Method and Apparatus for Controlling the Operation of a Diesel Engine" also discloses a system for limiting the amount of fuel admitted to the engine in dependence on engine speed and air mass flow data. That system makes additional corrections of the maximum amount of fuel based on air temperature and fuel temperature. Still further refinements include taking into account exhaust gas temperature and engine starting conditions. Indeed, a function generator is utilized for providing a maximum fuel quantity signal or value as a function of the engine speed and induction tube pressure. Although consideration is given to the appropriate function for providing maximum permissible fuel quantity associated with a particular speed and pressure while maintaining the exhaust below permissible smoke limits, that function becomes a preprogrammed constant in the function generator to be thereafter determined by engine speed and induction air pressure.
Existing control systems, although seeking to provide an optimized fuel limit or torque limit while maintaining engine operation within certain exhaust smoke limits, have heretofore relied upon a predetermined value determined as a function of existing engine conditions of speed, intake pressure, temperature and the like in an effort to attain that end. However, as certain characteristics of the engine change with time and wear, the predetermined functions may no longer be optimally suited to achieving the desired end. More specifically, changes in engine operating characteristics may result in the excessive emission of exhaust smoke with the passage of time. Alternatively, if the predetermined function is conservatively scaled to prevent such occurrence, the engine may not be permitted to operate at its maximum torque capabilities during the early stages of its life.